In Matric Addressed Liquid Crystal Displays, X data column lines and Y switching row lines are connected through thin film field effect transistors (FETs) to individual Liquid Crystal Display cells or pixels. In such a display the individual pixels are sequentially connected to their associated data lines as the field effect transistors are switched on from the switching lines.
Liquid Crystal Display devices, typically consist of a pair of flat panels of substrates sealed at their outer edges to form a chamber containing a Liquid Crystal material. Transparent electrodes (preferably indium tin oxide), are deposited on the inner surfaces of the two substrates in predetermined patterns. The interior surface of one panel is covered by a continuous transparent "ground or back plane" electrode while the interior surface of the opposite panel contains an array of individual transparent electrodes--referred to as "pixels" (picture elements)--configured in an XY matrix. The combination of the Liquid Crystal material, the pixel and back plane electrodes form capacitor-like cell structure between the two substrates. Application of electrical signals to the cells controls the ability of the individual cells to transmit light.
In operation, the orientation of the Liquid Crystal material molecules is controlled by voltages applied to the cell electrodes. The voltages affect the optical properties of the Liquid Crystal material thereby controlling the transmission of light through the cells and thereby the display of information. In a twisted nematic Liquid Crystal Displays crossed polarizer and analyzer elements are positioned on opposite sides of the substrates. Plane polarized light exiting from the polarizer passes through the cell, and its plane of polarization is rotated as it passes through the Liquid Crystal material. Application of voltage to the cell affects the rotation of the Liquid Crystal cell molecules. Below a threshold voltage known as "Off" voltage there is a 90.degree. twist of the Liquid Crystal molecules and a 90.degree. rotation of the plane polarized light so that essentially all of the light is blocked by the analyzer element. As the voltage increases above the "OFF" threshold, the degree to which the molecules are twisted is reduced thereby permitting a portion of the light to be transmitted until a second voltage threshold known as the full "ON" voltage is reached and the degree of twist is reduced to 0.degree. and essentially 100% of the light is transmitted. For voltages between the full "On" and full "Off" levels there are varying levels of light transmission and hence varying levels of brightness. Control of the Liquid Crystal cells to produce gray scale brightness levels is achieved by subdividing the cell voltage into increments between the full "On" and "Off" values.
LCD displays may also produce color images through the incorporation of color filter mosaics in registration with the individual pixel electrodes.
Although the instant invention will be described in connection with a twisted nematic Liquid Crystal Display, the invention is by no means limited thereto and is equally applicable to Guest/Host Displays containing a Liquid Crystal host material supporting one or more dichroic guest dyes.
To display video information in such X-Y Matrix Addressed Liquid Crystal Displays. It is necessary to energize the pixels so as to provide various levels of brightness to establish a gray scale between the "full-on" and the "full-off" states. To this end it is customary to digitize the analog video information in an A to D converter to represent the desired gray scale levels in digital form. The voltage between the "full-on" and "full-off" states is divided into increments to produce the desired number of gray scale brightness levels. The maximum possible number of brightness levels is desirable in order to achieve the best contrast and sharpness of detail. However, there is a practical limitation on the number of gray scale voltage increments that may be derived since the voltage range between the "full-on" and "full-off" states for the Liquid Crystal cell is relatively limited. Sixteen (16) level gray scale is most commonly used although thirty-two (32) and sixty-four (64) level gray scale would be desirable.
However, the transfer function of twisted nematic Liquid Crystal Display between the "full-on" and the "full-off" states (that is, the relationship between pixel voltage vs light transmission or brightness) is non-linear. Thus, even a sixteen (16) level brightness gray scale involves gray scale voltage increments as small as fifty (50) millivolts. Accurately maintaining fifty (50) millivolts increments over the operating temperature range is a difficult task. To provide thirty-two (32) level gray scale by a direct or "brute force" approach; that is by providing thirty-two (32) gray scale voltage increments would require substitution of 5-bit video conversion and driver hardware as well as a gray scale voltage generator and associated circuitry which is capable of generating and maintaining thirty-two (32) gray scale voltage increments some of which are twenty-five (25) millivolts or less over the temperature range. A need therefore exists for video conversion and data line driver circuitry which increases the perceived number of visual gray scale brightness levels without changing the 4-bit, sixteen (16) level hardware or the number of gray scale voltage increments. Specifically, the perceived visual gray scale levels must be increased to thirty-two (32) levels from sixteen (16) levels to improve image quality while utilizing 4-bit driver hardware and only sixteen (16) gray scale voltage increments.
Applicant has found that this highly desirable result may be realized by time multiplexing brightness levels of each pixel between adjacent levels during successive frames. At a frame refresh rate of 60 Hz the eye averages the brightness levels to produce an intermediate brightness level thus doubling the number of perceived gray scale brightness levels realizable with sixteen (16) gray scale voltage increments from sixteen (16) to thirty-two (32).